Electrodeionization (EDI) is a process for removing ions from liquids, such as water, by sorption of these ions to a material capable of exchanging these ions either for hydrogen ions (for cations) or hydroxide ions (for anions) and removal of the sorbed ions by the application of an electric field between an anode and a cathode.
A typical EDI module has at least one diluate compartment through which the liquid to be processed is passed and at least one concentrate compartment taking up the ions removed from the liquid in the diluate compartment. The diluate compartment is therefore often also called “product channel” and the concentrate compartment is therefore often also called “waste channel”.
At the outer edge of one side of the diluate compartment there is typically an anion permeable membrane which defines the outer limit of the diluate compartment on that side. The outer limit on the opposite side of the diluate compartment is typically defined by a cation permeable membrane. One or more concentrate compartments are formed on the opposite sides of the membranes from the diluate compartment. In particular, concentrate compartments are typically formed between the anion permeable membrane from the cathode side and the cation permeable membrane from the anode side. An anode side compartment (also called anode compartment) and a cathode side compartment (also called cathode compartment) is a compartment containing the anode or the cathode, respectively, and delimited by a membrane. Depending on type of membranes delimiting an electrode compartment (and type of neighbor compartment) the electrode compartment acts for decationization, or deanionization or concentration of an acid or a base. Thus, typically an anode compartment delimited by an anion permeable membrane concentrates an acid, while an anode compartment delimited by a cation permeable membrane becomes depleted in salt cations. Likewise, a cathode compartment delimited by a cation permeable membrane concentrates a base, while a cathode compartment delimited by an anion permeable membrane becomes depleted in salt anions.
The diluate compartment is filled with ion-exchange materials through which the water to be deionized flows. The ion-exchange materials in the diluate compartment selectively sorb the ions from the liquid upon exchange for hydrogen ions (for cations) or hydroxide ions (for anions). By the applied electric field, the sorbed anions are transferred towards the anode and the sorbed cations are transferred towards the cathode. Once they pass through the respective ion permeable membranes, they are passed into the concentrate compartment. The concentrate compartments may be filled with ion-exchange material or with inert liquid permeable material and a liquid flows through each concentrate compartment to thereby rinse it and transport the ions to waste.
The ion-exchange material employed in EDI modules is often made of polymer resin in the form of polymer beads, as described for instance in EP 1 282 463 B1. Alternative ion-exchange materials have been produced in the form of nonwoven or woven fabrics made of fibers that contain anion-exchange and cation-exchange functional groups, as described for instance in U.S. Pat. No. 6,423,205 and US 2006/0091013. A combination of ion-exchange resin beads and fabrics is described in WO 2005/011849 A.
Ion-exchange materials in the form of nonwoven or woven fabrics or porous blocks allow intensifying the purification process and porous blocks further allow simplifying the assembly of the modules. Such materials are typically produced by grafting of porous or fibrous substrate followed by chemical treatment thereby introducing ion-exchange functionality to the material. In the following, they are also simply called “grafted materials”.
Grafted materials allow further possibilities of special arrangements of ion-exchange materials inside compartments of an EDI module, which are hardly possible with a bed of ion-exchange resin beads.
Regarding the construction of a diluate compartment in EDI, the patent application JP 07-100391 A describes layers of ion-exchange nonwoven fabrics arranged in parallel with liquid flow, adjacent to a membrane of the same polarity and separated by a neutral open mesh screen, while in U.S. Pat. No. 6,423,205 the screen is supposed to have ion-exchange capacity. In case an ion-exchange screen is used, the water dissociation is electrochemically enhanced at the bipolar interface between the grafted materials oriented perpendicular to the electric field, and the generated H+ and OH− ions regenerate grafted materials and can be exchanged for ionic or ionizable contaminants thereby removed from water. In the EDI apparatus, as disclosed in FIG. 3 of U.S. Pat. No. 6,423,205, all layers are oriented parallel to the membranes and the liquid flow, the fibrous materials increase the active surface for ion removal, but since they exhibit resistance to flow compared to the open mesh screen, most of the flow streams through the channel between the fibrous materials filled by screen. In this case a long flow path is required to purify water to high resistivity. Together with the fact that compartments with layers parallel to membranes are relatively thin, e.g. 3 mm-5 mm, the assembly will require a relatively high membrane area per volume of water produced and result in relatively high material cost.
An improvement was disclosed in US 2006/0091013 where the layers of grafted materials are positioned perpendicular to the membranes and to the flow direction, as shown in FIG. 2 of US 2006/0091013. In this case thicker compartments can be used that reduces the pressure drop and material cost. Since all water can pass through the fibrous materials with high specific surface area, the ions should be removed efficiently in a relatively short path, which results in very good purification performance. The authors of US 2006/0091013 state that for achieving excellent purification performance it is important that water passes alternatively many times through the contacts between cation- and anion-exchange materials, typically 10 contacts between counter-polar layers of grafted materials per 1 cm of diluate channel length, which requires a complex equipment or extensive labor for cutting the layers and assembling the modules.
The electrochemically enhanced water dissociation with generation of H+ and OH− ions required for regeneration of grafted materials substantially occurs at the interface between grafted material and a counter-polar membrane, as illustrated in FIG. 5. The water dissociation could be enhanced on the interface between adjacent layers of grafted materials, but the orientation of the layers parallel to the electric field is not favored for this. Here an electrochemically enhanced water dissociation can occur due to a certain “roughness” of the interface between two fibrous materials and due to a deviation of the electric field lines from the parallel orientation between the electrodes, e.g. caused by different conductivities of materials over the length of the module. It is considered that the regeneration of materials with such orientation in the electric field may require a relatively high potential drop to be applied and may result in a relatively low energy efficiency for the deionization process.
As a further disadvantage of the technique described in US 2006/0091013, due to the orientation in a plurality of thin layers where H+and OH−ions are migrating in opposite directions, some of them can be driven by concentration difference to the interface between the counterpolar layers where they will recombine to water and thus will not be available for exchange with ionic and ionizable contaminants from water. In a similar way the salt ions, which are already removed from water and migrate inside corresponding ion-exchange material can recombine on the interface between counterpolar grafted materials and thus be rejected back into water, thereby decreasing the current efficiency of the purification process. The decrease of current efficiency through the recombination of H+and OH−ions to water and the rejection of removed salt anions and cations into water of the diluate compartment will be pronounced even stronger with an increase of the compartment thickness. Moreover, a considerable amount of the ions generated at the membrane/grafted material interface immediately pass through the corresponding membrane (H+ ion through the cation permeable membrane and OH− ion through the anion permeable membrane) and are forwarded directly into the respective concentrate compartment without participating in the regeneration of grafted materials. This further decreases the current efficiency and provokes strong local pH variations on the surface of the corresponding membrane in the concentrate compartment, which might be unwished due to the risk of scaling formation or materials degradation.
Object Of The Invention
An object of the present invention is to provide an EDI module benefiting from the advantages of flow-through ion-exchange materials in terms of an intensified purification process and enabling special arrangements of ion-exchange materials, which do however not suffer from the above-described drawbacks of the known EDI modules and apparatuses with regard to complexity of manufacturing and assembly of the modules, the flow-through channels between the ion-exchange material without sufficient interaction with the ion-exchange material and a decreased current efficiency.
A further object of the present invention is to provide an EDI apparatus which enables the purification of aqueous liquids for the production of water of high purity or ultra-pure water with unprecedented efficiency and at low costs of both manufacture and operation as well as a method for purifying a liquid, in particular water, with unprecedented efficiency and at low costs of operation and maintenance.
Still another object of the present invention is to provide a laboratory water purification system which can be readily used in a laboratory and provides purified water on demand.